Resin-coated metal plates or sheets, to which a resinous coating has been applied before delivery to users, can be roughly divided into two types: those designed for use without a top coating after forming so that the resinous coating can form the exterior of a product as such (hereinafter referred to as "precoated metal sheets"), and those designed for use with application of a top coating after press forming (hereinafter referred to as "preprimed metal sheets").
Precoated metal sheets having a topcoat layer, which make it totally unnecessary for users to perform a coating process, have been widely employed in the production of appliances and outdoor utensils and instruments. However, their use in automobiles has been limited, mainly for the reason that conventional precoated metal sheets have little or no electrical conductivity and cannot be bonded by welding, which is widely employed in assembling automobiles.
Automobile makers also greatly desire a simplified coating process. For this purpose, the following two approaches are conceivable. One is the use of precoated metal sheets (mainly steel sheets) to eliminate the entire coating process, and the other is the use of preprimed metal sheets to eliminate part of the coating process (electrocoating step, or electrocoating and intermediate coating steps of the coating process). The use of preprimed metal sheets, which can be put into practical use more easily than precoated metal sheets, will be realized initially in the automotive industry.
Among automotive parts to which prepriming can be applied, repair parts are considered to be most easily preprimed since they are coated individually and are composed solely of metal sheets (without using thick metal plates). On the other hand, prepriming of entire automotive bodies involves many problems, since application of prepriming thereto may require a change of bonding method and involves prepriming of small parts made of thick metal plates. Even for automotive repair parts, use of preprimed metal sheets has the merit of economy since it eliminates an electrocoating step or electrocoating and intermediate coating steps.
Many attempts have been made to develop resin-coated metal sheets, particularly resin-coated steel sheets, having various properties required for automotive materials, such as formability, corrosion resistance, and weldability. Regarding weldability, spot-weldable, resin-coated steel sheets have been developed in which spot weldability is afforded either by adding an electrically conductive pigment to the coated film or by reducing the thickness of the coated film by an extreme amount. Most of the resin-coated steel sheets which have been employed in automobiles are those comprising a base sheet of a plated steel sheet with a zinc- or aluminum-based plated coating, having thereon a lower chromate coating and an upper extremely thin organic resinous coating having a thickness on the order of 1 .mu.m (see, e.g., Japanese Patent Application Laid-Open No. 6-155658). These resin-coated steel sheets having a thin resinous coating, which may be called "organic-inorganic composite coated steel sheets", can be processed by welding or electrocoating while maintaining good corrosion resistance.
However, the good corrosion resistance of such a composite coated steel sheet indicates that corrosion resistance to pitting in a flat plane portion of the sheet before or after top coating is good, e.g., to a degree sufficient to guarantee prevention of pitting for ten years. On the other hand, precoated steel sheets are required to have corrosion resistance even on cut edge faces and punched edge faces (hereinafter referred to as "edge face corrosion resistance"), which is totally different from corrosion resistance to pitting on a flat plane.
Therefore, use of composite coated steel sheets having a thin resinous coating to fabricate automotive repair parts involves problems on the edge faces thereof. This is because conventional composite coated steel sheets have a very thin resinous coating with a thickness on the order of 1 .mu.m, which is insufficient for adequate protection of edge faces. Application of intermediate and top coating cannot cover the edge faces completely.
Accordingly, conventional composite coated steel sheets, even if intermediate and top coating is applied thereto without initial electrodeposition coating, do not have a sufficient level of edge face corrosion resistance. For this reason, they are considered to be highly corrosion-resistant steel sheets having corrosion resistance superior to plated steel sheets, and are mainly used as a steel sheet material for bodies of luxury cars by application of the entire coating process including electrodeposition coating, which does not lead to simplification of the coating process.
Automotive repair parts are coated with a repair paint for topcoating. Unlike the high-temperature curing coating compositions that are normally used in automotive fabrication, repair paint is a low-temperature curing coating composition usually baked at a temperature of from about 50.degree. C. to about 80.degree. C.
It is difficult for conventional composite coated steel sheets to achieve good adhesion to a coat formed from low-temperature curing repair paint.
The resin-coated metal sheets disclosed in Japanese Patent No. 1,711,387, for example, are not satisfactory with respect to edge face corrosion resistance and adhesion to low-temperature curing repair paint coating, although they possess good weldability, formability, and corrosion resistance on a flat plane.